Phase change heat transfer has been utilized in numerous applications from air conditioning to solar heating. Heat pipes are well known for employing phase transition and gravity for operability. However for the heat pipe, there is one drawback of contra-flow of vapor and liquid in the same pipe, which limits the transfer rate and the distance.
The loop heat pipe uses a chilled liquid that flows to the bottom of a solar collector. However, this technology introduces problems associated with the liquid level inside of the solar collector: if the liquid level is low, there can be a deficit of liquid in the upper part of the solar collector and, as a consequence, the temperature of the solar collector can rise to destructive levels, especially for a combination photovoltaic and thermal application. Also, when the liquid level is high, there can be a large flow of liquid out of the vapor pipe, preventing effective heat transfer.
Solar energy is still not quite competitive to traditional fossil energy in terms of return on investment (ROI). While the industry continues to strive to lower system cost (the denominator of ROI), another direction is to increase the return (the numerator of ROI).